The present invention relates to an improvement of an exhaust gas recirculation (EGR) system in an internal combustion engine for reducing NO.sub.x emissions in the exhaust gas.
In the known EGR system, a part of the exhaust gas is recirculated into the combustion chamber of the engine in order to lower the maximum combustion temperature so as to reduce NO.sub.x emission production. Such an EGR system comprises an EGR pipe which interconnects an exhaust pipe and an intake pipe of the engine, and an EGR valve mounted on the EGR pipe. The EGR valve operates in response to the engine temperature, vehicle speed, throttle valve opening or any other factors which indicate the driving condition of the vehicle, so as to control the EGR amount. An EGR cooler is arranged on the EGR pipe upstream of the EGR valve in order to avoid thermal damage to the EGR valve by exhaust gas of high temperature and to heighten the effect of the EGR system. However, in the light-loaded condition of the engine, that is, when the EGR amount is small, the EGR gas is over-cooled to or below 250.degree. C. This results in carbon accumulating and acid water condensing on the EGR valve and EGR pipe, causing the EGR ejection port to be clogged and the characteristic of the EGR valve to change. As a result, the known EGR system does not work satisfactorily and the desired EGR amount is not attained.